The present invention generally relates to a liquid crystal display (LCD), and more particularly, to a scan driving circuit and driving method for active matrix liquid crystal display (AMLCD).
An active matrix liquid crystal display includes an array of thin film transistors (TFT) formed on a panel, which is controlled by external column and row signals to display images. A block diagram for a typical display system of an active matrix liquid crystal display is illustrated in FIG. 1, in which a liquid crystal controller 201 is provided with display data and synchronization signal by a signal bus 101 and transmits the display data and synchronization signal to a data driver 202 by a data signal bus 206, a first line marker (FLM) 207 and a clock 208 to a scan driver 203 for the activation and operation of the scan driver 203, and a crystal alternating signal 209 to a power supply 204. The color tone voltages generated by the data driver 202 are transmitted to a liquid crystal panel 205 by a drain bus 210, and the select/unselect signal generated by the scan driver 203 for scan lines are transmitted to the liquid crystal panel 205 by a gate bus 211. Among the voltages generated by the power supply 204, select voltage level Vgon 212 and unselect voltage level Vgoff 213 are supplied to scan driver 203, and the opposite electrode voltage 214 of the liquid crystal panel 205 and color tone voltage 215 of the data driver 202 are supplied to the liquid crystal panel 205. The TFT liquid crystal panel 205 includes a matrix crossed by the drain bus 210 and gate bus 211, and at the cross point a cell is formed with a TFT switch 216 and a pixel liquid crystal 217. The gate of the TFT device 216 is connected with the gate bus 211, the drain of the TFT device 216 is connected with the drain bus 210, the source 218 of the TFT device 216 is one of the electrodes of the pixel liquid crystal 217, and the opposite electrode 219 of the pixel liquid crystal 217 is connected with an opposite electrode line 214.
The liquid crystal display controller 201 converts the display data and synchronization signal transmitted by the signal bus 101 to be the display data and liquid crystal driving signal for driving the TFT liquid crystal display, and transmits the display data and liquid crystal driving signal to the data driver 202 by the signal bus 206, the liquid crystal display driving signal to the scan driver 203 with the FLM 207 and clock 208, and the liquid crystal alternating signal 209 to power supply 204. The data driver 202 sequentially fetches the display data from the signal bus 206, and, when the display data of a scan line is completely fetched, converts the display data to the color tone voltage corresponding to the scan line, which is in turn outputted from drain bus 210, and the data driver 202 repeats the process for each scan line as such. Synchronized to the event that the data driver 202 outputs the color tone voltage to the liquid crystal panel 205 by the drain bus 210, the scan driver 203 sequentially applies the select voltage to the gate bus 211. When the select voltage Vgon is applied on the gate bus 211, the TFT device 216 in the liquid crystal display panel 205 enters the selected state and applies the color tone voltage upon the pixel liquid crystal 217 by the drain bus 210, such that the actual voltage applied upon the pixel liquid crystal 217 varies the twist angle of the liquid crystal to control the transmittance of light, thus implements color tone display. Furthermore, when the unselect voltage Vgoff is applied by the gate bus 211, the TFT device 216 in the liquid crystal display panel 205 enters the unselected state and keeps the voltage applied on the liquid crystal 217. Repeat the process during the interval of a frame, and all TFT devices 216 will be selected.
The resolution of an image is dependent on the number of the pixels, and since a scan line controls the on/off of a row of TFT devices, the more the pixels are in a TFT array, the more there are scan lines, and thus the number of pins of a liquid crystal display panel increase. As a result, it is harder to connect the scan driver to the liquid crystal panel. Besides the apparent difficulty of reducing the external circuit of a liquid crystal display, the prior art driver circuit is not formed directly on the panel, thus circuit integration and simplification cannot be achieved, and when a new technology has been developed, such as the case that low-temperature poly-silicon (LTPS) process is applied to fabricate the liquid crystal display, advantages cannot be exploited. With the increasing resolution and complexity of the liquid crystal display, those problems are getting even worse. Therefore, it would be desirable for a scan driving circuit and driving method to provide a large enough number of scan lines with less control signals.
One object of the present invention is to provide a scan driving circuit and driving method for active matrix liquid crystal display with a plurality of switching devices formed on the liquid crystal display panel and coupled between a plurality of scan lines and a plurality of control signal input ports, in which the number of the control signal input ports are less than that of the scan lines, and one set of control signals coupled to the control signal input ports from outside of the panel to manipulate the switching devices to drive the scan lines. Thus the number of the pins that the panel has to provide to drive the scan lines is reduced in order.
Another object of the invention is the utilization of Metal-Oxide-Semiconductor (MOS) devices of the same conductivity type serving as the switching elements to simplify the circuit and its fabrication process.
Still another object of the invention is the formation of the switching devices with low-temperature poly-silicon MOS devices so as to reduce the cost and difficulty of fabrication.
Yet still another object of the invention is to provide sequential or back-and-forth pulse signals as the driving waveform of the set of control signals.